The Climate System—the Water Cycle, Wind, and Energy Transport
In many simplified representations used to explain radiation processes , the impression is created that climate can essentially be explained by a radiative equilibrium.
In reality, however, the atmosphere is a highly dynamic flow system in which enormous amounts of energy are additionally absorbed, stored, transported, and reabsorbed through a wide variety of pathways and processes. This system includes:
- Evaporation of enormous quantities of water
- Convection (rising and sinking air masses)
- Latent heat (heat stored in water vapour)
- Advection (horizontal transport of energy and moisture by wind)
- Kondensation and Precipitation
⇒ These processes are not secondary—they are fundamental and central and summarized in the following chart.
(© Brugger 2026
The Underestimated Dominant Process
⇒ The climate system is regulated primarily by the water cycle, not by radiation.
Evaporation, transport of water vapor, and condensation are central processes of the global energy balance. Enormous amounts of energy are stored, transported, and released—not as sensible heat, but as latent heat.
This mechanism acts like a global balancing system:
- Energy is absorbed at the surface (evaporation)
- Transported over large distances (atmosphere/wind)
- Released elsewhere (condensation, cloud formation)
⇒ This process plays a decisive role in stabilizing the climate.
A clear illustration can be seen in the comparison between the tropics and deserts:
Despite very high solar radiation, temperatures in the tropics remain relatively stable throughout the year (See the example of Ecuador). The reason is not a lack of energy input, but a highly efficient removal of energy via the water cycle (Rainforests).
In contrast, dry deserts experience extreme temperature variations: very hot during the day due to solar radiation, and strong cooling at night
Evaporation
Evaporation from water surfaces refers to the phase transition from liquid water to water vapour below its boiling point. This process is a key component of the water cycle. According to the USGS, up to 90 per cent of atmospheric water vapour originates from the evaporation of oceans, seas, lakes and rivers. The remainder comes mainly from plant transpiration, with a very small proportion arising from sublimation.
In a water molecule (H₂O), the atoms are arranged in a bent geometry, resulting in a permanent dipole moment. This polarity facilitates the formation of hydrogen bonds, which cause water molecules to associate with each other. During evaporation, these intermolecular hydrogen bonds must be overcome.
The rate of evaporation from a water surface is influenced not only by air temperature but also by a complex interplay of multiple environmental factors.
- Solar radiation provides the energy for the phase transition
- The water temperature determines the saturated vapour pressure at the water’s surface
- A zone of high humidity forms above the water’s surface – the drier the air, the greater the vapour pressure gradient
- Wind carries the moist boundary layer away.
- The heat of evaporation required cools the water.
As long as the vapour pressure at the water’s surface is greater than that of the surrounding air, evaporation takes place. If the air immediately above the water is saturated and is not replaced, net evaporation largely comes to a standstill.
Why wind is so important
This thin, almost saturated layer of air, which forms directly above the water’s surface, halts further evaporation unless it is carried away.
We’re all familiar with this effect when air-drying laundry: laundry dries significantly faster on a windy day than when the air is still.
→ Wind:
- carries away the water-vapour-saturated air,
- brings in drier ambient air,
- increases turbulent mass transfer,
- and, in strong winds, can significantly enhance evaporation.
Evaporation over water surfaces is therefore a coupled process of energy and mass transfer. Whilst temperature influences the potential vapour pressure at saturation, it does not alone determine the actual rate of evaporation.
Wind not only enables the horizontal transport of the water vapour produced, but also sustains evaporation itself by removing the moist boundary layer. A change in the air flow near the ground can therefore influence both the local evaporation rate and the spatial distribution of water vapour.
Orders of Magnitude of Energy Flows
To understand the significance of these processes, it is useful to consider the scale of energy flows in the water cycle:
- Approximately 13,000 km³ of water are present in the atmosphere. Multiplied by the latent heat of evaporation, this corresponds to about 32,000 exajoules (EJ) of latent heat.
- Atmospheric water is cycled 38 to 39 times per year, corresponding to a heat transport of roughly 1,250,000 EJ — about one-third of the annual incoming solar energy.
- Even a very small change in global water vapor circulation—e.g., only 2.5% — would mean that approximately 32,000 EJ of energy is no longer transported from the Earth’s surface into the atmosphere.
In comparison:
To increase the temperature of all atmospheric CO₂ by 1 °C would theoretically require about 2.8 EJ of energy.
⇒ These figures clearly show:
Energy flows associated with the water cycle are orders of magnitude larger (factor ~11,000) than the purely thermal storage capacity of trace gases.
Conclusion
What follows from this?
⇒ The Earth releases its energy through a complex, dynamic system in which:
- radiation, mass, absorption and desorption
- high-energy air flow and heat transfer
- and, above all, the water cycle
are inseparably linked. It should also be recognised that atmospheric flows form part of closed circulation systems.
Air transported from one region to another must be balanced by corresponding return flows. Otherwise, a deficit of air mass would develop in the region of origin, while an excess of air mass would accumulate in the destination region.
The commonly simplified representation of the greenhouse effect therefore falls short when it focuses primarily on radiative processes.
⇒ The decisive factor for the stability and dynamics of the climate is the water cycle as the dominant energy transport mechanism and the wind is an essential component of this system.
